This invention relates to measurement of bore diameters and more particularly to bore measuring devices which are capable of measuring large, gradual variations in the bore size.
A multi-legged apparatus called a star gage has historically been used to measure bore diameters. The star gage was essentially an internal micrometer which was expandable in the radial direction to meet the bore's wall. Its travel in the radial direction was limited to approximately 1/4 inch and was highly dependent on operator skill for accuracy of result. Additionally, the star gage was useful only for substantially constant diameter, straight bores.
Turbines and other devices using rotatable members such as shafts are typically manufactured with straight, substantially constant diameter bores, but if a defect such as a crack or impurity inclusion is found in the rotor's bore during post manufacturing inspection thereof, the defect is often machined away by cutting into the shaft and radially enlarging the bore. To avoid stress concentrations, the bore diameter was gradually expanded in the radial direction from the normal bore to the enlarged bore so as to form a so-called "bottled bore" whose profile is suggestive of a tapered bottle outline. Measurement of such bottle bores has typically been accomplished during formation of the bottle bore by radially swinging the defect-removing cutting tool into the coincidental axis of an associated cutting machine and bore, withdrawing the cutting machine and tool from the bore, swinging the cutting tool radially outward, and measuring the distance from the axis of the cutting machine to the radially outer extent of the cutting tool. During routine shut-downs at or maintenance on rotors, turbine shafts, or other bored members, the bore diameter must be inspected and measured very accurately and compared to the post-manufactured bore diameter to give early indications of various failure modes such as impending "creep" failure. During such inspection, the cutting tool bore measurement procedure is, of course, not well suited since it is undesirable to enlarge the bore beyond that which is necessary to eradicate the defect.
At periodic intervals of operation the apparatus (such as a turbine) utilizing the bored rotor is typically shut down, disassembled, and inspected with a view toward routine and preventive maintenance. Inspection and measurement of the bore during such a shutdown is considered desirable since changes in the bore diameter provides an indication of potential future failure. The length of the shutdown must be minimized to maximize the use of the turbine and revenue obtained as a result thereof. It is thus incumbent on the inspector to make the bore measurements as accurately as possible and do so in the minimum time which is consistent with safety.